I was late for a United flight several months ago; when I boarded the plane, they did not scan my boarding pass. They still found a seat for me, but without scanning my boarding pass, United assumed I didn't make the flight. My flight from LAX back to Chicago was cancelled and I had to fight for two hours on the phone with United customer care to convince them that, yes, I indeed did fly their equipment to LAX and still should permitted to fly my return ticket home.